The invention relates to an electromagnetic positioning system for an object free to move with respect to a fixed reference system in which the displacement of the object is of the order of a few centimeters, with a precision better than 1 millimeter.
The invention is used for applications in many domains, and particularly in the structural mechanics domain to study deformations of solids subject to stresses, or for protection to monitor the position of an object that could be stolen or in the virtual reality domain to position elements with a man-machine interface.
At the present time, there are many systems for positioning a mobile object with respect to a fixed reference system. These include some systems that are dedicated solely to short distance positioning, in other words displacement of the object that moves only a short distance with respect to the reference system, in other words of the order of a few centimeters to a few meters.
One of these short distance positioning systems is a visual technique that consists of fixing a recognizable target on the mobile object and aiming at this target with several cameras. Images from the cameras are then processed to determine the position of the target and consequently the position of the object. However, these systems are limited by the fact that the target has to be seen by the cameras, in other words it must be within the field of vision of these cameras. Therefore, no obstacles can be accepted. Furthermore, this system has a large external infrastructure, in other words it is large; therefore, it is difficult to use it for positioning within a few centimeters.
A variant of this visual technique consists of using telemeters, for example laser telemeters that aim at the target. This technique is used to position an object moving over distances of a few centimeters; but it always has the disadvantage that the target must be visible from the telemeter.
A mechanical technique can also be used to position an object at short distance, this technique consisting of fixing the object to a set of mechanical links. In this case, the link between the object to be positioned and the reference system is mechanical, which has disadvantages in the sense that not all displacements may be authorized, and detection of displacement is not very precise.
Another technique for short distance positioning consists of using magnetic dipoles associated with magnetometers. Patent application EP-0-215 695 describes a system of this type. Positioning of magnetic dipoles by magnetometers is adapted to positioning of objects moving over distances varying from a few centimeters up to a few meters; the system needs to be miniaturized to detect displacements of a few centimeters, in other words for very short distance positioning. However, it is difficult to miniaturize magnetometric sensors for which the precision must be high.
Furthermore, positioning an object in three dimensions makes use of complex calculation algorithms; therefore it is difficult to envisage the use of this type of system to position an object moving within a few centimeters in three dimensions.
The purpose of the invention is to overcome the disadvantages of the techniques described above. Consequently, it proposes an electromagnetic system for positioning a mobile object with respect to a fixed reference system, in which the displacement of the object is of the order of a few centimeters and the precision is better than 1 millimeter.
More precisely, the invention relates to a system for positioning a mobile object with respect to a fixed reference system, characterized by the fact that:
the mobile object is equipped with a probe (1) comprising at least one resonant circuit enabling reception of a magnetic field with frequency fg and transmission of a magnetic field at frequency fe; and,
the reference system (2) is provided with:
means of transmission/reception of a magnetic field comprising a voltage generator (G) generating a signal with frequency fg powering at least one electromagnetic coil (L3; L5) and at least one capacitor (C3; C5) forming a series resonant circuit, with the said coil;
anti-dazzle means by which the transmission/reception means can distinguish the magnetic field with frequency fe from the magnetic field with frequency fg; and
detection and comparison (4) means to detect the amplitude of the voltage induced at the terminals of the resonant transmission/reception circuit and to deduce the displacement of the probe with respect to the reference system.
In a first embodiment, the transmission frequency fe of the magnetic field and the reception frequency fg of the magnetic field are equal, the probe comprising a single parallel resonant LC circuit. In this case, the magnetic field transmission/reception means may include resistances connected to the resonant transmission/reception circuits and to the detection and comparison means, using a bridge set up.
In another embodiment, the probe comprises a first parallel LC resonant circuit for reception of the magnetic field with frequency fg and a second series LC resonant circuit for transmission of the magnetic field with frequency fe, where fgxe2x89xa0fe.
According to another embodiment of the invention, the positioning system is characterized in that the transmission/reception means comprise:
a resonant transmission circuit for the magnetic field at frequency fg; and
at least one reception circuit for the magnetic field at frequency fe comprising a secondary coil connected to a capacitor to form a secondary resonant circuit.
Advantageously, the axes of the secondary coils are parallel to each other and parallel to the axis of the probe coil.
The detection and comparison means may comprise:
a differential amplifier connected to the terminals of each resonant reception circuit;
synchronization means for synchronizing signals output from each differential amplifier; and
processing means to deduce the position of the probe starting from signals originating from secondary resonant circuits.
According to a first variant, the anti-dazzle means are electromagnetic coils placed close to each secondary coil and powered by the generator.
According to a second variant, the anti-dazzle means are differential amplifiers into which the signal output from the detection and comparison means is applied to one input and an adjustable voltage is applied to another input.